----- Original Message -----
> From: "tj" <tj@xxxxxxxxxx>
> To: "CAI Qian" <caiqian@xxxxxxxxxx>
> Cc: "Al Viro" <viro@xxxxxxxxxxxxxxxxxx>, "Linus Torvalds" <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Dave Chinner"
> <david@xxxxxxxxxxxxx>, "linux-xfs" <linux-xfs@xxxxxxxxxxxxxxx>, "Jens Axboe" <axboe@xxxxxxxxx>, "Nick Piggin"
> <npiggin@xxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx
> Sent: Wednesday, October 5, 2016 11:30:14 AM
> Subject: Re: local DoS - systemd hang or timeout (WAS: Re: [RFC][CFT] splice_read reworked)
>
> Hello, CAI.
>
> On Wed, Oct 05, 2016 at 10:09:39AM -0400, CAI Qian wrote:
> > > This one seems to be the offender. cgroup is trying to offline a
> > > cpuset css, which takes place under cgroup_mutex. The offlining ends
> > > up trying to drain active usages of a sysctl table which apprently is
> > > not happening. Did something hang or crash while trying to generate
> > > sysctl content?
> >
> > Hmm, I am not sure, since the trinity was running from an non-privileged
> > user which can only read content from /proc or /sys.
>
> So, userland, priviledged or not, can't cause this. The ref is held
> only while the kernel code is operating to generate content or
> iterating, which shouldn't be affected by userland actions. This is
> caused by kernel code hanging or crashing while holding a ref.
Right, the trinity calls many different random syscalls and options on those
/proc/ and /sys/ files and generate lots of different errno. It is likely
some of error-path out there causes hang or crash.
CAI Qian
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